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Author Topic: JBL Horns Richard Hamilton Phenomenal Film Handler Posts: 1341 From: Evansville, Indiana Registered: Jan 2000 posted 05-23-2010 04:24 PM                     When I took a sound reinforcement course, I was taught that phase was the alignment of the drivers. Polarity is the connections. The instructor demonstrated the phasing by stacking 2 bookshelf speakers and running pink noise. He slowly slid the top speaker back and there was an immediate dip on the analyzer screen. As he moved the speaker further back, the dip moved along with it.  |  IP: Logged Michael Voiland Expert Film Handler Posts: 219 From: Naperville, IL US Registered: Aug 2009 posted 05-24-2010 01:32 AM                     Interesting that is true. Polarity would be the correct term to use I guess. It doesn't matter the theaters have all been so botched and haven't been eqed in 6 years at least. This is including the new screen some new curtains more decorations (yuck) I am just trying to keep this stuff from breaking because everything at the place i work is run into the ground then fixed sort of.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 05-24-2010 09:25 AM                     The flaw in just changing the polarity is that it presumes a periodic wave form and that you are out only for 1/2 of a cycle. Mechanical time alignment is best but failing that, electrically delaying the component furthest forward is best. In Rane analog crossovers, like the AC22, you can adjust the delay continuously. Thus, play a tone at the crossover frequency and adjust the delay until the output is maximum...it is very fast to do. Steve  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 05-24-2010 09:32 AM                     Rick, Your teacher demonstrated time alignment... not phasing! He probably didn't realize or know what he was doing. If drivers are too close to each other you can experience phase cancellation and you will see that dip where the frequencies cancel out. But you also want drivers in the correct plane so the signals all arrive to you at exactly the same time. When you reverse the polarity of a driver you are causing it to operate 180 degrees out of phase. Like Gord says... use a phase checker and before hand know which way your system's components are supposed to be connected. You haul the ladder around considerably less and make less trips up to the screen speakers if you do. Mark  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 05-24-2010 10:16 AM                        A polarity change is effectively the same as a frequency-dependent delay line that delays by 1/2f seconds. Consider an all-pass filter, which has a transfer function H(s) = (sRC-1)/(sRC+1), i.e. a pole-zero pair on the real axis, mirrored across the imaginary axis. At low frequencies, the phase shift of the transfer function is 180 degrees, is 90 at 1/2piRC, and 0 at high frequencies. If we keep moving the quadrature frequency higher, RC approaches zero, and more frequencies experience the 180 degree phase shift. At RC=0, H(s) = -1, or a simple polarity reversal. Hocus-pocus aside, a polarity reversal is genuinely useful in crossovers where the effective phase of the driver outputs in the crossover region causes cancellation, but is not useful for fixing the drivers' physical alignment. In the crossover situation, there is only a small region of frequencies where both drivers play, so you don't end up with comb filtering. If you had two identical drivers playing the same signal, you would get comb filtering if they were physically offset, and switching the polarity on one would only move the comb, whereas a real time delay (by moving drivers or electrically with an all-pass filter of some sort) would eliminate it.  |  IP: Logged Richard Hamilton Phenomenal Film Handler Posts: 1341 From: Evansville, Indiana Registered: Jan 2000 posted 05-24-2010 10:36 AM                     Syn-aud-con Mark, I think they know what they're teaching. It's possible I'm remembering wrong. Mark, this is a good page to read. phasing and polarity  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 05-24-2010 11:08 AM                        Time-delay, physical shifting of sources, the effect of crossovers on signals, and polarity all relate to phase. The confusion comes in because phase is the positional relationship between two pure tones. Complex signals don't have a single phase, but their pure tone components do. You can look at a plot of phase versus frequency just as you would a plot of magnitude versus frequency on a spectrum analyzer, although it wouldn't be terribly useful in practice. So when you're talking about phase, you're talking about an abstract overarching concept. A time delay or physical shift has a phase response that is proportional to the frequency, so it would show up as a sloped straight line on our hypothetical phase spectrum analyzer. The polarity reversal would be a straight line at 180 degrees. And a real crossover would have a curve of some sort (the same you'd see in a Bode plot).  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 05-24-2010 12:50 PM                     Markertek sells a great tester for only $100, check it out HERE. It is well-made in metal cases and is about the only one around anymore. Pulse unit can drive line level or a speaker directly and the receiver has a built-in mic, and it also has built-in cable testers for 1/4" as well as XLR. A great value and a MUST for setting up any sound system and checking speaker polarity.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-24-2010 03:01 PM                     I beleive it was a german firm that made the one smart sold  |  IP: Logged

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